This invention relates to permanent magnet, direct current motors, and more precisely to an improved stator design and method of manufacture for such motors.
Permanent magnet, direct current motors contain a stator or magnetic frame surrounding the armature rotatably mounted within the frame. The stator supports at least two permanent magnets disposed in very close proximity to the outer edge of the laminated armature core. These magnets define a pole pair with the flux passing from one magnet through the armature windings to the other magnet. The stator is constructed of a low reluctance magnetic material and completes the previous magnetic flux circuit from the magnets. Hence, a magnetic circuit analogous to an electrical circuit is traceable from one magnet through the armature windings and returning through the stator to the first magnet.
When constructing small permanent magnet motors it is necessary to achieve high power density and it becomes increasingly critical to optimize the efficiency of the stator magnetic circuit. To achieve this, the air gap between the magnets and the armature should be as small as possible. Air is of high reluctance and, therefore, produces significant flux loss across the gap. In addition, the stator should be of low reluctance material and offer uniform reluctance. This requires a stator construction devoid of air gaps. Air gaps can be produced by broken or sectionalized walls and raise the local reluctance, which produces flux loss and inferior motor performance. On the other hand, by adherence to these criteria it is possible to maintain the same flux density with even smaller magnets and a somewhat smaller stator, without any deterioration in motor performance.
Nevertheless, it becomes expensive and difficult to accomplish these ends in a small motor, particularly those of the type produced in high volume, for example windshield wiper and blower motors. The small size makes it difficult to achieve uniformily small air gaps between the magnets and armature and small stator normally can be viewed as being inherently less rigid or sturdy and, therefore, permits greater air gap variations throughout the life of the motor, not to mention during manufacture. This can produce wide variations in motor performance. It thus becomes necessary to reinforce the stator, which obviously occasions an unnecessary increase in the size and cost of the motor. At the same time, it is preferred to avoid magnetic coupling between the armature shaft and stator where the shaft passes through the armature, which can produce flux loss.